This invention relates generally to wind turbine energy systems and more particularly to pitch control systems for wind turbines.
In one known wind turbine, a pitch control system having a fully regenerative silicon controlled rectifier (SCR) bridge drives a 4.2 KW series DC motor. This type of system has been used in servo motor drives for many years and is commercially available. The SCR drive has the advantage of simplicity, but may not be able to deliver the level of pitch system performance that may be needed in newer and/or larger wind turbines.
In the event of a loss of AC input power, at least one known wind turbine system pitches the blades of the wind turbine using emergency pitch batteries. The blades are pitched to a position that would prevent blade overspeed. The AC voltage drop is sensed by the pitch control system and the emergency pitch system is activated. The wind turbine control system modulates the emergency pitch system and attempts to keep the hub rotational speed below overspeed limits. In many cases, the turbine control issues a fault and stops the turbine. However, known wind turbine systems use DC link capacitors and an H bridge power converter circuit, and do not have the ability to pitch the blades using this circuit once the small amount of energy stored in the DC link capacitors is depleted.